1. Field of the Invention
The present invention relates to semiconductor chip assembly, and more particularly to a semiconductor chip assembly with a semiconductor device and a ceramic/metal substrate and its method of manufacture.
2. Description of the Related Art
Semiconductor devices such as packaged and unpackaged semiconductor chips have high voltage, high frequency and high performance applications that require substantial power to perform the specified functions. As the power increases, the semiconductor device generates more heat. Furthermore, the heat build-up is aggravated by higher packing density and smaller profile sizes which reduce the surface area to dissipate the heat.
Semiconductor devices are susceptible to performance degradation as well as short life span and immediate failure at high operating temperatures. The heat not only degrades the chip, but also imposes thermal stress on the chip and surrounding elements due to thermal expansion mismatch. As a result, the heat must be dissipated rapidly and efficiently from the chip to ensure effective and reliable operation. A high thermal conductivity path typically requires heat conduction and heat spreading to a much larger surface area than the chip or a die pad it is mounted on.
Light emitting diodes (LEDs) have recently become popular alternatives to incandescent, fluorescent and halogen light sources. LEDs provide energy efficient, cost effective, long term lighting for medical, military, signage, signal, aircraft, maritime, automotive, portable, commercial and residential applications. For instance, LEDs provide light sources for lamps, flashlights, headlights, flood lights, traffic lights and displays.
LEDs include high power chips that generate high light output and considerable heat. Unfortunately, LEDs exhibit color shifts and low light output as well as short lifetimes and immediate failure at high operating temperatures. Furthermore, LED light output and reliability are constrained by heat dissipation limits. LEDs underscore the critical need for providing high power chips with adequate heat dissipation.
LED packages usually include an LED chip, a submount, electrical contacts and a thermal contact. The submount is thermally connected to and mechanically supports the LED chip. The electrical contacts are electrically connected to the anode and cathode of the LED chip. The thermal contact is thermally connected to the LED chip by the submount but requires adequate heat dissipation by the underlying carrier to prevent the LED chip from overheating.
Packages and thermal boards for high power chips have been developed extensively in the industry with a wide variety of designs and manufacturing techniques in attempts to meet performance demands in an extremely cost-competitive environment.
Plastic ball grid array (PBGA) packages have a chip and a laminated substrate enclosed in a plastic housing and are attached to a printed circuit board (PCB) by solder balls. The laminated substrate includes a dielectric layer that often includes fiberglass. The heat from the chip flows through the plastic and the dielectric layer to the solder balls and then the PCB. However, since the plastic and the dielectric layer typically have low thermal conductivity, the PBGA provides poor heat dissipation.
Quad-Flat-No Lead (QFN) packages have the chip mounted on a copper die pad which is soldered to the PCB. The heat from the chip flows through the die pad to the PCB. However, since the lead frame type interposer has limited routing capability, the QFN package cannot accommodate high input/output (I/O) chips or passive elements.
Thermal boards provide electrical routing, thermal management and mechanical support for semiconductor devices. Thermal boards usually include a substrate for signal routing, a heat spreader or heat sink for heat removal, pads for electrical connection to the semiconductor device and terminals for electrical connection to the next level assembly. The substrate can be a laminated structure with single layer or multi-layer routing circuitry and one or more dielectric layers. The heat spreader can be a metal base, a metal slug or an embedded metal layer.
Thermal boards interface with the next level assembly. For instance, the next level assembly can be a light fixture with a printed circuit board and a heat sink. In this instance, an LED package is mounted on the thermal board, the thermal board is mounted on the heat sink, the thermal board/heat sink subassembly and the printed circuit board are mounted in the light fixture and the thermal board is electrically connected to the printed circuit board by wires. The substrate routes electrical signals to the LED package from the printed circuit board and the heat spreader spreads and transfers heat from the LED package to the heat sink. The thermal board thus provides a critical thermal path for the LED chip.
Conventional packages and thermal boards have major deficiencies. For instance, dielectrics with low thermal conductivity such as epoxy limit heat dissipation, whereas dielectrics with higher thermal conductivity such as epoxy filled with ceramic or silicon carbide have low adhesion and are prohibitively expensive for high volume manufacture. The dielectric may delaminate during manufacture or prematurely during operation due to the heat. The substrate may have single layer circuitry with limited routing capability or multi-layer circuitry with thick dielectric layers which reduce heat dissipation. The heat spreader may be inefficient, cumbersome or difficult to thermally connect to the next level assembly. The manufacturing process may be unsuitable for low cost, high volume manufacture.
In view of the various development stages and limitations in currently available packages and thermal boards for high power semiconductor devices, there is a need for a semiconductor chip assembly that is cost effective, reliable, manufacturable, versatile, provides flexible signal routing and has excellent heat spreading and dissipation.